Could be caused by interferences. Things you can try:

• Select 20 MHz channel width.
• Set 802.11n-only mode.
• Use another channel (make a WLAN survey to find out best channel).

One problem with exchanging antennas is the EIRP - make sure you don't violate the max. EIRP (not a technical issue, but a legal issue).

I don't know what beam width your yagi antennas have, but even directional antennas are sensitive to interferences, especially on channel 9, which is used by many other customer devices such as DECT phones, bluetooth, garage door controls, TV transmitters, remote controls , microwave ovens etc. Such devices can't be detected since they don't send a SSID.

Next problem is the signal strength of the remote client (your friend's laptop). This could explain low throughput because there is no free line of sight leading to high attenuation of the signal of your friend's laptop and the distance of 40m is pretty much at the limit for 802.11b/g (~70m for 802.11n) for antennas of a laptop, even if there is a free LoS. Same is true for smartphone antennas.

In my experience directional links in 2.4 GHz band behave much worser than in the 5 GHz band (depends on the country you're living in). For example, on a 5 GHz directional link over 600 meters in Europe I reach up to 85 Mbps full-duplex data throughput, which is pretty good in a 802.11n WLAN (wireless speed 300 Mbps yields ~210 Mbps data speed half-duplex in 802.11n, which gives 105 Mbs full-duplex max. throughput in theory). See this story here - it's a directional link with Pharos devices, not an EAP-Outdoor, but it demonstrates the difference between the 5 GHz vs. 2.4 GHz band. I did first tests with 1x pair of 5 GHz CPEs and 1x pair of 2.4 GHz CPEs simultaneously, then replaced them by 2x pairs of 5 GHz CPEs. Using 2.4 GHz devices on this directional link the throughput did go down to 3-4 Mbps just due to interferences (~140 APs inside the 65º antenna beam width, all competing for AirTime).

Even EAP225-Outdoor mesh networks use a 5 GHz channe for connecting the nodes of the mesh. Maybe, two meshed EAP225-Outdoor could increase throughput dramatically on your link compared to one EAP110-Outdoor and a laptop used indoor. Or you try two CPE510, which would fit better for just a P2P link if no outdoor omni antennas are needed at all.